PLASTIDS 



69 



form no starch at all, the products of synthesis being compounds of other 

 kinds (p. 62). In Anthoceros it is reported that the outer elements of 

 the group of "pyrenoid bodies" develop starch while the inner ones 

 multiply by division. In Selaginella also, the starch appears to develop 

 from such proteinaceous bodies (R. Ma, 1930). 



Origin and Multiplication. — The differentiated plastids seen in 

 mature tissues may be traced back to plastid primordia, or proplastids, 

 in the young cells of the meristem or embryo." These appear in the 

 cytoplasm of living cells as minute bodies grading off to the limit of 

 visibility.^® In the leaf they enlarge, sometimes becoming rod shaped, 

 and develop chlorophyll (Fig. 33) when they are still no more than 0.5/i 

 in diameter. Starch grains can be detected inside of them with polarized 



Fig. 33. — Development of chloroplasts from their primordia (proplastids) in living meso- 



phyll of Zea Mays. (After Randolph, 1922.) 



light (Zirkle, 1927a). They may be markedly altered by fixing reagents, ^^ 

 fluids containing potassium bichromate often causing them to elongate 

 and acetic acid destroying them altogether; later on they become more 

 resistant to the latter reagent. After osmication or silver impregnation 

 they may appear like hollow structures with a blackened periphery. 



At all stages up to maturity chloroplasts can be seen to divide, 

 although constriction often occurs without resulting in division (Kass- 

 mann). Division generally seems to occur in either of two ways: by 

 simple constriction or through the formation of a hyaline cleavage zone. 

 The process may occupy many hours or even several days. 2° Few, if any, 

 proplastids are left by the time the leaf-cell becomes fully differentiated. 

 In the cells of the root tip the development of the leucoplasts is much the 

 same, except for the absence of chlorophyll. Their division appears to be 



'^ Pensa, Lewitsky, Guilliermond, Dangeard, Emberger, Noack, Friedrichs, 

 Mottier, Randolph, Kassmann, Kiyohara, Bowen, Zirkle, Weier, Motte, and others. 



^* Randolph (1922) on Zea, Kassmann (1926) on Cabomba (chondriosomes ?, 

 see p. 85 of this book). Bowen (1929a) disagrees for Vicia. 



1^ Zirkle (1927a), Kiyohara (1927, 1930), Weier (1931a). 



20 Heitz (1922), Kiyohara (1926). 



